In mobile radio systems, radios can be equipped with transmitters that operate on two different frequency bands. When the radio operates in one frequency band, the radio transmits directly to another mobile radio in the communication network. In the alternative frequency band, the radio transmits to a base station used in the communication network.
When a mobile radio communicates directly with another radio, it does so by transmitting in the so-called "talk-around" frequency band. Alternatively, when the radio communicates with the base station, it does so by transmitting in the so-called "normal" frequency band. For example, in an 800 MHz mobile radio system, a mobile radio will transmit to the base station in a normal frequency band that covers 806-824 MHz. Alternatively, when the radio communicates directly with other radios, it transmits in the talk-around frequency band covering 851-869 MHz. Of course, these normal and talk-around frequency bands are not the only possible frequency bands available for normal and talk-around transmissions. For example, a 900 MHz mobile radio system uses 896-902 MHz as the normal frequency band and 935-941 MHz as the talk-around frequency band. Other normal and talk-around frequency bands can also be employed.
In FIG. 1, a plurality of mobile radios, radio 1, radio 2, . . . radio n (10, 12, 14) communicate with each other in a mobile radio communication system including base station 16. In normal communications, when radio 1 (10) communicates with radio n (14), radio 1 communicates its message to base station 16 in the normal frequency band, and base station 16 relays the message to radio n in the normal frequency band.
Under certain conditions, such as when radio 1 is no longer proximate to base station 16, interference prevents radio 1 from communicating with base station 16, or some other situation prevents or hampers communication between radio 1 and base station 16, then radio 1 can switch from the normal frequency band to the talk-around frequency band to communicate directly with radio n (14). In the talk-around situation, the base station 16 is out of the communication loop, such that radio 1 and radio n communicate directly with each other in the talk-around frequency band.
For radios that have talk-around capability, circuitry must be provided within their transmitters to switch between the normal frequency transmission mode and the talk-around frequency transmission mode. That is, when the talk-around conditions are imposed or detected, radio 1 (10) must include circuitry in its transmitter to switch from transmitting in the normal frequency band to transmitting in the talk-around frequency band. Prior art circuitry for accomplishing that transition from normal frequency transmission to talk-around frequency transmission (and viceversa) is shown in FIGS. 2 and 3.
As can be seen by comparing FIGS. 2 and 3, the transmitter circuit for these two prior art transmitters is identical, with the exception of the frequency ranges employed by them. FIG. 2 relates to a 800 MHz mobile radio and FIG. 3 relates to a 900 MHz mobile radio.
In FIG. 2, a transmitter circuit 15 is shown in three stages, the modulation circuit 20, the transmission modulation phase lock loop circuit 21, and the band select circuit 22. Within the modulation circuit 20, the input signals I and Q, which are desired to be transmitted, are input to an I and Q modulator 23 where they are mixed with an intermediate frequency signal from the transmission IF local oscillator 24. The output of the I and Q modulator 23 (IF MOD OUT) will be the I and Q signals modulated to the intermediate frequency. From there, the modulated intermediate frequency signal is input to a band pass filter 25 to remove out-of-band spurs and to provide the reference signal S.sub.R at the intermediate frequency.
In the transmission modulation phase lock loop circuit 21, a phase detector 26, loop filter 27, transmission voltage control oscillator 28, amplifier 29, and coupler 30 are provided in a forward path, and an attenuator 31, mixer 32, main local oscillator 33, and band pass filter 34 are provided in a feedback loop. Both the transmission voltage control oscillator 28 and the main local oscillator 33 have dual operating characteristics. That is, when the radio is operating in the normal frequency mode (i.e., communicating with the base station 16), the transmission voltage control oscillator 28 operates in a frequency band from 806 to 824 MHz and the main local oscillator 33 operates at a frequency in the range of 806 plus the intermediate frequency of local oscillator 24 through 824 plus the intermediate frequency of local oscillator 24 MHz, i.e., the normal frequency band incremented by the intermediate frequency. Alternatively, in talk-around mode, when the radio communicates directly with another radio, the TX voltage control oscillator 28 operates in a frequency range of 851 to 869 MHz, and the main local oscillator 33 operates in a range of 851 plus the intermediate frequency from local oscillator 24 through 869 plus the intermediate frequency of the local oscillator 24 MHz, i.e., the talk-around frequency band incremented by the intermediate frequency.
The operation of the transmission modulation phase lock loop circuit 21 is as follows: when the phase lock loop is locked, the transmission voltage control oscillator 28 operates at the normal frequency band from 806 to 824 MHz. The signal from voltage control oscillator 28 is then amplified by amplifier 29 and supplied to coupler 30. Coupler 30 outputs the resulting signal at the RF Mod Out point on FIG. 2, as well as on the coupled output of coupler 30, as signal S.sub.C.
On the feedback path, S.sub.C is attenuated in attenuator 31 and then mixed in mixer 32 with a signal from the main local oscillator 33. Since the transmitter is operating in normal frequency mode (i.e., the phase look loop is locked), the main local oscillator 33 is operating at the normal frequency band incremented by the intermediate frequency from the local oscillator 24. The output of the mixer 32 (after passing through band pass filter 34 for filtering) will be a feedback signal S.sub.F at the same intermediate frequency as was provided at the output of the band pass filter 25 (signal S.sub.R).
Both S.sub.R (from modulation circuit 20) and the feedback signal S.sub.F (from the feedback loop) are input to the phase detector 26. The phase detector 26 compares the feedback signal S.sub.F with the reference signal S.sub.R to generate an error voltage. The error voltage passes through the loop filter to generate the control voltage for the transmission voltage control oscillator. As a result, the transmission voltage control oscillator 28 has the same modulation as the reference signal S.sub.R but operated at the desired transmit frequency.
When the radio transitions into the talk-around mode, the operation of the modulation phase lock loop circuit 21 is as described above, except that the transmission voltage control oscillator 28 and the main local oscillator 33 transition into different operational characteristics. In the talk-around mode, the transmission voltage control oscillator 28 operates at the talk-around frequency band (851 to 869 MHz in FIG. 2) and the main local oscillator 33 operates in the frequency band of (851 plus the intermediate frequency from the local oscillator 24) through (869 plus the intermediate frequency from the local oscillator 24) MHz. Otherwise, the operation is the same as described with respect to the normal mode above.
The transmission modulation phase lock loop circuit 21 is only operational for phase modulation with a constant envelope. Thus, the circuit shown in FIG. 2 is not adaptable to other kinds of modulation other than constant envelope phase modulation.
Significantly, the transmission modulation phase lock loop circuit 21 employs a main local oscillator 33 which must be operational at two different frequency band characteristics: the normal frequency band incremented by the IF frequency and the talk-around frequency band incremented by the IF frequency. Similarly, the transmission voltage control oscillator 28 must also be capable of operating in two different frequency band characteristics: the normal frequency band and the talk-around frequency band. Because these oscillators must be capable of operating in dual operational modes, the oscillators are larger and more costly than single characteristic oscillators.
The final stage of the transmitter 15 is the band select circuit 22. The band select circuit 22 includes two RF switches 35 and 38 and two RF filters 36 and 37. The RF modulation output from the coupler 30 is provided to the band select circuit 22 at the RF switch 35. Depending upon the position of the RF switch 35 (either normal mode or talk-around mode), the RF switch 35 directs the RF modulation output signal to either RF filter 36 (for normal transmission mode) or RF filter 37 (for talk-around transmission mode). RF filters 36 and 37 are adapted to filter high and low frequencies on either side of, respectively, the normal frequency band (filter 36) or the talk-around frequency band (filter 37). The signal from the filter 36 or 37 is then provided to RF switch 38 for output to the amplifier 39 and ultimately to the antenna for transmission to either another radio 12, 14 (in talk-around transmission mode) or the base station 16 (in normal transmission mode). RF switches 35 and 38 operate in tandem such that the RF modulation output signal from the coupler 30 is connected to the amplifier 39 through only one of the two RF filters 36 and 37. That is, the RF modulation output signal will pass either through the normal transmission path (filter 36) or the talk-around transmission path (filter 37) as a result of the operation of the RF switches 35 and 38.
FIG. 3 is identical to FIG. 2, except that the operation of the transmitter 15 is designed for a 900 MHz mobile radio. In the embodiment of FIG. 3, the normal frequency band is 896 to 902 MHz and the talk-around frequency band is 935 to 941 MHz. Accordingly, the transmission voltage control oscillator 28 operates in two modes, normal and talk-around, at these respectively frequency bands. Similarly, main local oscillator 33 also operates in two frequency bands, normal and talk-around, at the respective normal and talk-around frequency bands incremented by the intermediate frequency of the local oscillator 24. In addition, RF filter 36 in the normal transmission path operates at the normal frequency band and the RF filter 37 in the talk-around transmission path operates in the talk-around frequency band. Otherwise, the operational characteristics of the transmitter 15 in FIG. 3 are identical to those described with respect to FIG. 2 above.
Unfortunately, the transmitter circuits 15 of FIGS. 2 and 3, which implement the prior art embodiments, require a voltage control oscillator 28 that operates with two different frequency band characteristics and a main local oscillator 33 which also operates with two frequency band characteristics. That is, these oscillators must be switchable from the normal frequency band operation to a talk-around frequency band operation, depending upon whether the transmitter 15 is operating in the normal or talk-around mode. Such dual characteristic oscillators are disadvantageous because they are bulky and expensive. In addition, the transmitters 15 in FIGS. 2 and 3 have limited use with only constant envelope phase modulation. Other kinds of modulation cannot be used in the FIG. 2 and FIG. 3 prior art embodiments.